Proof beyond reasonable doubt is proof enough, save for philosophers who won't be satisfied by anything much.
"assume," something to fill in the gaps."
It's not so much an assumption but a plausible explanation that is used as conjecture. As investigations fail to refute it, it gains credibility and is then accepted as truth, and a theory. Once accepted, by and large, it stays and further discoveries tend to modify, and only rarely totally replace. Much better that saying, "A deity did it".